1. Field of the Invention
The present invention relates to a microfluidic device, and more particularly to a microfluidic device motivated by centrifugal force that has an improved flow control of fluid on its flowing into channels by adjusting burst frequencies of capillary valves.
2. Description of the Prior Arts
Due to developments in medicine, pharmacy, biotechnology and environmental monitoring, overwhelming chemical analysis and related devices and technicians are required. However, the general public needs a more convenient and simpler analytical process without being limited by technical knowledge, devices and occasions.
With progresses of microelectronic techniques and semiconductors, great efforts have been devoted to the development of efficient, sensitive, precise and miniature automatic detection techniques in the field of biological analysis and biomedical diagnostics. The concept of Micro Total Analysis Systems (μTAS) was proposed in the early 1990s. Merely one μTAS is capable of including sample preparation, chemical reaction, separation and purification of, and detection and analysis of analyte as a complete chemical analytic process. Thus, μTAS satisfies the need for a more convenient and simpler analytical process.
Miniature of μTAS is beneficial in that it is easy to carry. Use of microelectronic components in μTAS lowers electricity consumption and reduces cost. Moreover, μTAS requires smaller amounts of samples or reagents, resulting in decrease of expenses on reagents. Furthermore, during procedures of an automatic chemical process, flow rate, amount of materials and sequence of reactions in each procedure profoundly affect the results of the analysis. μTAS is regarded as a minimized batch chemical process. A major focus of studies in μTAS is microfluidic technique. The microfluidic techniques encompass various fluidic functions, such as valving, mixing, metering, splitting and separation.
Microfluid is driven by various methods, including mechanical micropumps and non-mechanical micropumps. The former includes peristaltic pump, ultrasonic pump and centrifugal pump. The latter includes pumping by electrical, magnetic, and gravity forces. In the case of the centrifugal pump, it is used in disc type microanalytical system, also called microfluidic disc system. Microfluidic disc system motivates fluid flow by centrifugal force and controls fluid flow by using passive capillary valve. The underlying mechanism of passive capillary valve is that capillary pressure difference or Laplace pressure difference prevents fluid flow. Therefore, fluid flow can be regulated by manipulating the balance between centrifugal force and capillary pressure. The critical rotational frequency, corresponding to the centrifugal force which overcomes the capillary pressure, is called burst frequency.
As for capillary valves in microfluidic system, currently a lot of related techniques have been published. U.S. Pat. No. 6,143,248 discloses that capillary pressure is associated with the arrangement, geometry and surface characters of capillary valves and reservoirs, and quantitative transferring of fluid is achieved under a related principle. In 2001, Anderson et al. modifies a portion of a microchannel by inductively-coupled plasma (ICP) with hydrophobic materials to form a hydrophobic surface on a portion of the microchannel. The change of the surface property produces a valving effect called hydrophobic valve. In 2003, Feng et al. disclose that hydrophobic valve can also be made by self-assembled monolayers (SAMs) by changing the geometry of channel to produce valve effect. In 2006, Cho et al. adopt annular channels and rectangular channels in capillary valving, propose a model of capillary valves with different angles of opening (60°, 90° and 120°) and verify predicted burst frequencies with experimental results. In 2006, Kwang et al. suggest that capillary valving is useful for microfluidic control process and further illustrate that fluid flow can be controlled by capillary valve through the changes of geometry and surface property of microchannels.
However, the aforesaid references only propose control of fluid flow with changes in geometry and surface modification and how to predict burst frequency. None of them reveals the relationship between positions, arrangement or orientation of capillary valves in the microfluidic system, especially the significance of positions proximal to the center of the microfluidic disc to fluid flow control. Moreover, almost all current microchannels are arranged at positions with a larger radial on the microfluidic disc because more microchannels can be implemented. Under those designs, the burst frequencies for the valves are usually lower than 2000 RPM. Since the burst frequencies of the capillary valves at positions with various radial distances are limited to lower than 2000 RPM, they tend to overlap each other. Therefore, current techniques of burst valves have disadvantages of unable to effectively release fluid in correct sequence.
To overcome the shortcomings, the present invention provides a microflluidic device to mitigate or obviate the aforementioned problems.